In the applicant's U.S. Pat. No. 4,052,648, entitled "Power Factor Control System for A.C. Induction Motors", there is disclosed a power reduction system for induction motors in which the operating power factor of a motor is monitored, and the effective voltage input to the motor is controlled as a function of the difference between a commanded power factor signal and the operating power factor. In the parent application, the applicant disclosed a power factor type control system particularly adapted for three-phase induction motors.
In addition to minimizing motor power consumption during normal running conditions, which was the goal of referenced patent application, a second area of concern is that of current surges which occur when a motor is first turned on. These surges can amount to five to eight times normal operating current and are both wasteful of electrical power and can contribute to excessive load surcharges being borne by electrical users. While discrete starting circuits have been employed which ideally gradually apply a starting voltage, it has been found that in some instances, these circuits fail to provide a smooth start-up, and not infrequently there may occur significant motor vibrations during at least a portion of the start-up interval.
It is the object of this invention to provide a single control system which both regulates power usage during normal running of a motor and provides for a graduated turn-on of motor voltage without the attendant problem of motor vibration. It is a particular object of the invention to provide a system of this category for a three-phase motor where graduated voltage starting circuits are most frequently used.